Generally, coated sheet of paper are manufactured by coating an inorganic pigment such as clay, calcium carbonate, aluminum hydroxide (Al(OH)3), titanium oxide (TiO2), etc. on paper, where a natural binder such as casein, starch, etc. or an artificial binder such as a styrene-butadiene latex, polyvinyl alcohol, acrylic latex, etc. is used for the adhesive since the above inorganic pigment does not have adhesive force. Presently, artificial synthetic binders are used mainly owing to their advantages in that it is easier to adjust their physical properties and it is more convenient to use them compared to natural binders. Among them, styrene-butadiene latexes that are not expensive but have a superior performance are used typically. When manufacturing a coating solution, various kinds of additives such as dispersion agents, thickner, insolublizer, etc. are used along with an inorganic pigment and a binder. Still, the most important chemicals are an inorganic pigment and a binder, which should be selected to obtain balanced physical properties of coated paper.
The most frequently used inorganic pigments are clay and calcium carbonate. Clay has a lamellar-type structure and is advantageous in that it is possible to obtain a high paper gloss and printing gloss but is disadvantageous in that it has a low fluidity but a large amount of demand for a binder. Whereas, in case of calcium carbonate, it is advantageous in view of its fluidity, adhesive force, ink acceptance, paper brightness, opacity, etc. but is problematic in that it is required to have a much greater chemical stability of the coating solution for the calcium ion.
Recently, there has been a movement to confront with the improvement of productivity and supply of printed matters by increasing the coating speed as the speed of manufacture of paper has been faster gradually. The coating speed of recent has been improved to be fast up to about 1,000˜1,500 m/min. Such a fast coating speed implies a greater shearing force during coating, and therefore, the mechanical stability of latexes becomes more important. It is also required to have a high chemical stability of latexes since the amount of use of calcium carbonate is increased as the performance of calcium carbonate is increased while reducing the prime cost.
As described in the above, the stability of latexes may be classified into the chemical, mechanical, and thermal stability. And all of these three types of stability should be secured in order to secure a high-level stability.
It has been demanded more seriously to improve the adhesive force of binders as there has been recently a strong trend of reducing the content of a binder for improvement of the paper quality and cost reduction when manufacturing coated paper. Along with the above, it has been demanded greatly to improve the ink-drying speed due to a fast printing speed. If only these two physical properties are improved, it is possible to obtain coated paper that conform to the manufacturing and printing processes of coated paper of recent.
The adhesive force of coated paper is expressed while a binder to be inputted into the coating solution goes through the drying process and films are formed. The better the films are formed, the greater the adhesive force becomes. However, the ink-drying speed is lowered as the amount of air bubbles in the coating layer is reduced although the adhesive force is increased. It implies that the adhesive force and the ink-drying speed are somewhat inversely proportional physical properties, and therefore, it is very difficult to improve both of them by simple adjustment of the glass transition temperature, etc. of the latex.
As a result of studies conducted by the inventors of the present invention in order to improve simultaneously the adhesive force, ink-drying speed, air permeability, and stability of styrene-butadiene latexes, it is found that the above object may be achieved by controlling the gel content and molecular weight of the outermost layer of the shells in the multiple core-shell structure. In other words, it is found that it is possible to simultaneously improve the adhesive force, ink-drying speed, air permeability, and stability of latexes for paper coating by employing a general method of control of the gel content and molecular weight by inputting a chain transfer agent along with monomers for manufacturing of shells during the process of polymerization in order to coat the core with the shells and adjusting the gel content and molecular weight of the outermost layer of multiple shells by inputting a fixed amount of the chain transfer agent singly for a fixed amount of time at a proper rate of conversion after inputting of those monomers. The present invention is completed based on this finding.